In general, a power transmission device used as a center differential for four-wheel vehicles includes a housing having a plurality of housing components fixed by bolts, and a planetary gear mechanism and a differential gear mechanism which are disposed on a rotational axis of the housing. A drive torque inputted from an engine into the planetary gear mechanism is transmitted to the differential gear mechanism through the housing and further to the front and rear wheel sides from one pair of side gears of the differential gear mechanism (see Japanese Patent Unexamined Publication (Kokai) No. Hei 7-117516).
The conventional power transmission device is large in part number, complicated in construction, and heavy in weight. Therefore, it is demanded that the number of parts be decreased.